1. Field of Invention
The present invention relates in general to mixed analog and digital signal processing and in particular, to sample rate converters with minimal conversion error and analog to digital and digital to analog converters using the same.
2. Background of Invention
In many applications, converting data from its native analog form into the digital domain for processing, storage and transmission provides the best overall system performance. One well known example is audio processing where analog audio is digitized through analog to digital (A/D) conversion and then processed, for example filtered or compressed, and then stored on a digital storage medium such as a compact disk (CD) or digital video disk (DVD). On playback, the digital data is decompressed, as required, reconverted to analog through digital to analog (D/A) conversion, and finally presented to the end user as audible tones.
According to the Nyquist Theorem, so long as the analog waveform is sampled during A/D conversion at a sampling frequency at least twice as high as the highest frequency component, that waveform can be successively reconstructed during subsequent D/A conversion. In actual practice, oversampling A/D and D/A converters are typically used because of their relative ease in implementation. For example, in an 8x oversampling converter operating on data with a base sampling rate of 44.1 kHz, the data are sampled at a rate of 352.8 kHz. At the higher sampling rate, operations such as anti-aliasing filtering are easier since a substantial amount of the noise power is translated to frequency bands well above the band of the signal of interest.
Sample rate conversion is an additional problem which must be addressed when processing digitized analog data. For example, professional digital audio is typically recorded with a sampling rate of 48 kHz while typical playback devices operate with a base sampling rate of 44.1 kHz. Sample rate conversion, and specifically down-conversion, is therefore required to ensure that the recorded audio properly plays back. There are several existing sample rate conversion techniques, including decimation for lowering the sampling rate and interpolation for increasing the sampling rate. Notwithstanding, these techniques are still subject to some significant disadvantages including the need for substantial silicon area for fabricating the requisite interpolation/decimation filters, as well as limitations on the ability to convert to fractional sampling rates.
The principles of the present invention are demonstrated in conjunction with sample rate converters which can suitably be integrated into analog to digital and digital to analog converters and other applications. According to one particular embodiment, a sample rate converter is disclosed for converting a data stream having a first base sampling frequency to a data stream having a second base sampling frequency which includes up-sampling circuitry for receiving first oversampled data having a first oversampling ratio with respects to the first base frequency and outputing second oversampled data having a second oversampling ratio with respects to the first base sampling frequency. Resampling circuitry is included for resampling the second oversampled data by a resampling frequency ratio of integers representing a ratio between the first and second base frequencies and generating in response third oversampled data having the second oversampling ratio with respects to the second base frequency. Finally, in this particular embodiment, down-sampling circuitry is provided for down-sampling the third oversampled data to generate fourth oversampled data having the first oversampling ratio with respects to the second base frequency.
Among other things, the principles of the present invention allow for the performance of sample rate conversions with little or no conversion error based on the designer""s choice. Moreover, when integrated within digital to analog converters and analog to digital converters, sample rate converters according to the inventive principles can perform such minimal error conversions without introducing aliasing into the baseband of the signal.